What Is Predicativism?

The official slogan of predicativism is what Russell called the “vicious circle principle”: no class can contain elements which are definable only in terms of the class itself. Another way to say this is that we do not accept any entity which cannot be defined without referring to a class to which it belongs. It is hard to make this principle precise, but the underlying sentiment is that the elements of any collection should be understood as in some way logically preceding the collection itself. Operationally, this means that the comprehension axiom